ignoring case in sed command

can anybody tell me a command to get sed command which ignores case.

example is like this
I want to search a file for the pattern "shameem" and this should replace the occurence of "SHAMEEM" also with "shammu".

cat file1 | sed "s/shameem/shammu/" > file1

so if the file1 consists of
"shameem gouse lazam SHAMEEM"

then this command should give the output as

"shammu gouse lazam shammu"

please help me in this reg...
Thank you
Shameem S

$ echo "SoMe TeXt"| sed 'y/ABCDEFGHIJKLMNOPQRSTUVWXYZ/abcdefghijklmnopqrstuvwxyz'/
some text

But this is a more elegant solution (nt using sed):

$ echo "SoMe TeXt" | tr '[A-Z]' '[a-z]'
some text

I am sorry this wont work. because it changes whole text. I dont want to change the remaining text other than matching pattern.
for e.g.

shameem GOUSE Lazam

I want to change this as

shammu GOUSE Lazam

but not

shammu gouse lazam

I want to replace shameem with shammu. Nothing else.
Thank you for your reply. Its a good solution for conversion from uppercase to lower case or vice versa.

if you have any idea about my issue let me know...

I misunderstood.

Ok, new try:

$ SAMPLE="ShAmeEm GOUSE Lazam"
$ echo $SAMPLE
ShAmeEm GOUSE Lazam

$ echo $SAMPLE | sed 's/shameem/Shammu/i'
Shammu GOUSE Lazam

$ SAMPLE="SHAMEEM GOUSE Lazam"
$ echo $SAMPLE
SHAMEEM GOUSE Lazam

$ echo $SAMPLE | sed 's/shameem/Shammu/i'
Shammu GOUSE Lazam

Whatever the upper/lowercase combo of the searchstring (shameem) it will be replaced by Shammu.

BTW: What does 'Shameem/Shammu GOUSE Lazam'' mean??